var arr1 = [1,2,3];
var arr2 = [4,5,6,7];
var newArr = [];

function fn(a1,a2) {
  var n = a2.length;
  for (var i = 0 , j = 1; i < n; i++) {
    a1.splice(j,0,a2[i]);
    j += 2;
  }
  newArr = a1;
  console.log(newArr);
}
fn(arr1,arr2);
